Quote from OOTP (The Eye of the Snake): "Harry dreamed he was back in the DA room. Cho was accusing him of luring her there under false pretences; she said he had promised her a hundred and fifty Chocolate Frog Cards if she showed up. Harry protested . . . Cho shouted, 'Cedric gave me loads of Chocolate Frog Cards, look!' And she pulled out fistfuls of Cards from inside her robes and threw them into the air.

Then she turned into Hermione, who said, 'You did promise her, you know, Harry . . . I think you'd better give her something else instead . . . how about your Firebolt?' And Harry was protesting that he could not give Cho his Firebolt, because Umbridge had it, and anyway the whole thing was ridiculous, he'd only come to the DA room to put up some Christmas baubles shaped like Dobby's head . . ."

Observation: Cho and Hermione both appeared in Harry's ridiculous dream. Ridiculous as it is, I believe there's a sort of sign. Note that Cho turned into Hermione. I believe this is a sign of a future transition from Cho to Hermione. It is also a transition from childish puppy love to a more mature love. Note that in the dream, Cho was pretty childish, screaming about Harry's promise of Chocolate Frog Cards and then she turned into a mature Hermione, who calmly suggested he (Harry) give her(Cho) his Firebolt.

Quote from OOTP (The Second War Begins): "Harry looked around. Cho was passing, accompanied by Marietta Edgecombe, who was wearing a balaclava. His and Cho's eyes met for a moment. Cho blushed and kept walking. Harry looked back down at the chessboard just in time to see one of his pawns chased off its square by Ron's knight. 

'What's - er - going on with you and her, anyway?' Ron asked quietly. 

'Nothing,' said Harry truthfully. 

'I - er - heard she's going out with someone else now,' said Hermione tentatively. 

Harry was surprised to find that this information did not hurt at all. Wanting to impress Cho seemed to belong to a past that was no longer quite connected with him; so much of what he had wanted before Sirius's death felt that way these days . . . the week that had elapsed since he had last seen Sirius seemed to have lasted much, much longer; it stretched across two universes, the one with Sirius in it, and the one without. 

'You're well out of it, mate,' said Ron forcefully. 'I mean, she's quite good-looking and all that, but you want someone a bit more cheerful.' 

'She's probably cheerful enough with someone else,' said Harry, shrugging."

Observation: So there! Harry's no longer interested with Cho which is probably (and hopefully) the start of my theory on the transition from Cho to Hermione.